1. Field of the Invention
The present invention relates to a frequency modulation circuit used for communication devices such as mobile phones and wireless LAN devices, and more particularly to a frequency modulation circuit which is capable of outputting a highly precise frequency-modulated signal regardless of variation in a characteristic of a VCO.
2. Description of the Background Art
Communication devices such as mobile phones and wireless LAN devices are required to secure precision of an output signal and operate with low power consumption. Such communication devices are required to have a frequency modulation circuit for outputting a highly precise frequency-modulated signal. Hereinafter, conventional frequency modulation circuits will be described.
There has been a conventional frequency modulation circuit which uses a PLL circuit, thereby correcting an output frequency of a VCO. However, such a frequency modulation circuit is not always capable of outputting a highly precise frequency-modulated signal due to variation in a characteristic of the VCO. FIG. 8 shows an example of variation in a VCO characteristic of the conventional frequency modulation circuit. In the conventional frequency modulation circuit, in order to obtain, e.g., a predetermined output frequency fo when VCOs respectively having different characteristics as shown in FIG. 8 are used, signals Va, Vb and Vc are required to be used as control signals for the VOCs.
There has been a conventional frequency modulation circuit for automatically correcting such variation in a VCO characteristic, e.g., frequency modulation circuit 50 disclosed in the Japanese Laid-Open Patent Publication No. 2-162407 (hereinafter, referred to as Patent Document 1). FIG. 9 is a block diagram showing an exemplary structure of the conventional frequency modulation circuit 50 disclosed in Patent Document 1. As shown in FIG. 9, a VCO 51 changes a frequency of an output signal D (i.e., frequency-modulated signal) in accordance with a control signal C. A phase comparing section 52 detects a phase difference between the output signal D from the VCO 51 and an input signal A, and outputs the detected phase difference as an error signal B. A correction circuit 53 detects a deviation, from a predetermined value, of the frequency of the output signal D of the VCO 51, and outputs the detected deviation as a correction signal E. An addition section 54 adds the error signal B to the correction signal E, and outputs a resultant signal as the control signal C.
FIG. 10 shows an exemplary characteristic of the VCO 51, which has been corrected by the conventional frequency modulation circuit 50. As shown in FIG. 10, the conventional frequency modulation circuit 50 is able to obtain the predetermined output frequency fo by using a single input signal A even if variation in the characteristic of the VCO 51 is caused by outputting, from the correction circuit 53 to addition section 54, of correction signals E which are Va, Vb and Vc, respectively. However, as shown in FIG. 10, the conventional frequency modulation circuit 50 does not correct a gradient of the characteristic of the VCO 51, and when the output frequency of the VCO 51 changes, the conventional frequency modulation circuit 50 is not always able to output a highly precise frequency-modulated signal.
Patent Document 1 discloses a conventional frequency modulation circuit 60 which is capable of correcting the gradient of the characteristic of the VCO 51. FIG. 11 is a block diagram showing an exemplary structure of the conventional frequency modulation circuit 60. The conventional frequency modulation circuit 60 in FIG. 11 is capable of correcting the gradient of the characteristic of the VCO 51 by outputting, from a multiplying section 55 to the addition section 54, a correction error signal H which is a result of multiplying the error signal B by the correction signal E.
However, the conventional frequency modulation circuit 60 corrects the variation in the characteristic of the VCO 51 under the assumption that the gradient of the characteristic of the VCO 51 is fixed. In reality, however, the gradient of the characteristic of the VCO 51 is not fixed. In other words, the VCO 51 does not have a linear sensitivity. FIG. 12 shows an exemplary input/output characteristic of the VCO 51 included in the conventional frequency modulation circuit 60. As shown in FIG. 12, the VCO 51 has a highest sensitivity when a control voltage Vt is at a predetermined voltage value Vtx, and the greater the deviation of the control voltage Vt from the predetermined voltage value Vtx, the lower is the sensitivity. Thus, even though the conventional frequency modulation circuit 60 is capable of correcting the gradient of the characteristic of the VCO 51, the frequency modulation circuit 60 is not capable of correcting the non-linearity of the sensitivity of the VCO 51.